I though Vicky's representation of slavery to be pretty reasonable. Well except that the only colored images in the whole game were slaves. All other pops, even Africans, are white. Also what wasn't reasonable was work output. Slaves did produce at comparable levels. What Vicky should have done was keep the militancy for slaves rising especially in American slaves. French slaves (New Orleans) had much different conditions. They could work professionally and have businesses, they just paid a portion to their 'owner' and when they purchased their freedom they lived well. This changed radically when the Louisiana territory became part of the US. Not all slavery is the same.
Slaves in Arab and African nations also had different conditions. Slaves held by American Indians eventually earned their membership in the tribe, it was more a process of assimilation of the captured, the original ancient purpose of ancient slavery.
All other discussions of wage earner's being slaves are just political perspective or propaganda.
The decision for slaves to not consume resources and have money was erroneous. They in fact did eat and have higher levels of consumption than basic needs as slave owners found that they still had to provide incentives.
American (US) slavery was especially odious in practice as it mainly focused on one 'race' as a slave race and that you could be 'born' a slave.
Slavery is distinct from indentured servitude.
Serfdom could or could not be inherited. Depends on the tradition or law. Mainly it was about peasant mobility. It still exists in Russia today. Yes. It does. But it isn't as strictly enforced as it was under communism. (no joke)
I do know a lot of people who lived under communism and yes they did feel like slaves and surfs.
There still does't seem to be any way to directly end slavery in a country.
I think for the game some simple modes would be reasonable. Slave, surf, employee, business owner/aristocrat/commissars.
A pop in America would likely own its farm (RGO) and thus not need an 'aristocrat' for better efficiency, that would be directly related to the literacy and experience of the pop.
Commissars would be a population not a leader, unique to communist governments replacing priests and aristocrats, they would insure party loyalty.
I would also replace Aristocrats and capitalists pops with 'leaders', just like the military units.
So each combination situation would have different affecting factors. But some would be exclusive. Only aristocrats(commissars

) could be associated with surfs.